In the formation of wood pulp from which paper is made, one commonly-employed procedure involves chemical treatment of wood chips to dissolve lignin and free the cellulosic fibres from the wood chips in a digestion operation, such as by the Kraft process. The resulting slurry of wood fibres in spent digestion chemicals, or black liquor, then is passed to a brown stock washing operation wherein the pulp is washed to remove the black liquor, which contains a variety of sodium salts and lignin. The washed pulp then usually is passed to a bleach plant wherein the pulp is bleached and further purified to provide the desired product.
It is desirable to remove the black liquor as efficiently as possible from the pulp in the brown stock washing operation, so as to minimize the chemical requirements of the bleach plant. At the same time, it is desirable to minimize the volume of wash water employed, so as to minimize dilution of the black liquor as it passes to the pulp mill recovery system, which typically involves an initial evaporation operation. The greater the dilution of the black liquor, the greater is the evaporative heat and hence the energy requirement of the evaporation operation.
Brown stock washing generally is carried out by a displacement washing operation wherein a screened drum (typically 15 feet in diameter and 16 feet long) rotates about a horizontal axis through the pulp slurry to pick up a mat of the slurry on its screened surface and then showers of wash water are applied to the outer surface of the mat to displace the black liquor from the pulp mat through the screen into the interior of the drum, usually with the assistance of vacuum applied internally of the drum. The brown stock washing operation generally comprises a plurality, typically three or four, of such displacement washing operations, with wash water flowing countercurrent to the pulp between the individual displacement washing operations and the pulp mat being repulped between the individual pulping operations.
The volume of wash water required to displace a unit volume of liquor entrained in the pulp determines the efficiency of the displacement washing operation. If a single unit volume of wash water is required, then the washing is 100% efficient. However, pulp mill displacement washing operations never attain such efficiency levels, for example, four times the volume of wash water is required to achieve a 75% washing efficiency.
This inefficient use of wash water arises from a phenomenon known as "Channelling", whereby channels for wash water flow are formed through the pulp mat during displacement washing, so that the wash water tends to try to flow through the channels, rather than uniformly through the pulp mat, the channels providing the lesser resistance to wash water flow. As a consequence, some of the liquor trapped inside the fibres is not displaced by wash water during the washing operation.
This phenomenon is discussed in U.S. Pat. No. 4,297,164 and in an article in Tappi Journal, November 1984, pp. 100 to 103. The solution to the channelling problem which is suggested in this prior art is to alter the rheological properties of the wash water, so that the mobility of the wash water is decreased. In particular, the prior art describes the addition of polymers that affect the permeability of the wash water with respect to the pulp mat, when the wash water begins to penetrate a region of high permeability in the pulp mat (i.e. a channel), the region becomes filled with the wash water which has a lover mobility than the solution to be displaced and, in this way, the channeling penetration in retarded and a reduction in wash water requirements is realized. U.S. Pat. No. 4,810,328 similarly discloses compositions including anionic polymers.